urjtag will search bfin-elf-as, bfin-uclinux-as, bfin-linux-uclibc-as in $PATH and call it to assemble the instructions. So one of these commands is required to be available in $PATH to use this assembly statements in bfin execute command. bfin-elf-objcopy, bfin-uclinux-objcopy or bfin-linux-uclibc-objcopy is also required.

If GAS has not supported the the assembly statements you want to execute, or because other reasons, you cannot use assembly statement, you can use encoded instructions.